The present invention relates to an arc welding apparatus that solely has a constant current characteristic output means and a constant voltage characteristic output means.
A prior art arc welding apparatus construction is shown in FIG. 2.
In FIG. 2 are shown a transformer 1 for welding, a current control circuit 2 for controlling the secondary side output of the welding transformer 1 by means of a current control device, a holder 5 for manual welding, a welding rod 6 for manual welding, a constant current characteristic output terminal 8, a constant voltage characteristic output terminal 9, an output terminal 10 for welding base metal, a torch 11 for a CO.sub.2 arc welding apparatus, a base material 12 for welding, a constant current characteristic coil 13 and a constant voltage characteristic coil 14. The constant current characteristic coil 13 and the constant voltage characteristic coil 14 have been wound around separate iron cores 13a and 14a, respectively, as shown in FIG. 2.
This prior art arc welding apparatus operates to control an output voltage from the welding transformer 1 by means of the current control circuit 2 and supply an output from the current control circuit 2 to the constant current characteristic output terminal 8 via the coil 13 and to the coil 14 via the constant voltage characteristic output terminal 9.
This prior art enables manual welding by connecting the manual welding holder 5 to the constant current characteristic output terminal 8 and enables CO.sub.2 arc welding by connecting the CO.sub.2 arc welding apparatus torch 11 to the constant voltage characteristic output terminal 9. The prior art is constructed so that the coil 14 operates when performing CO.sub.2 arc welding and the coil 13 operates when performing manual welding.
FIG. 3A shows a constant current characteristic of a power source for use in securing constant depth of penetration into the base metal while maintaining a welding current constant regardless of a change in arc length according to a welding method such as manual welding and TIG (Tungsten Inert Gas arc) welding, which cannot automatically adjust the arc length. Assuming now that the arc length is L1 as shown in the figure in the case of, for example, manual welding, the operating point is located at S1 in FIG. 3A that shows a relation between a welding current and a welding voltage. In this case, when the arc length extends to L2 due to hand movement or the like, the arc voltage increases, consequently moving the operating point from S1 to S2. However, with the power source of the constant current characteristic of FIG. 3A, the welding current scarcely varies in accordance with the shift of the operating point to S2, and therefore, welding can be continued while securing the constant depth of penetration into the base metal. If a power source of a constant voltage characteristic as shown in FIG. 3B is used, then the welding current sharply decreases in accordance with the shift of the operating point to S2, so that not only the securing of constant depth of penetration but also the continuation of welding become difficult.
On the other hand, according to CO.sub.2 arc welding and MIG (Metal Inert Gas arc) welding by which a welding wire is automatically fed, a power source of the constant voltage characteristic shown in FIG. 3B is used so as to continue satisfactory welding by keeping the welding voltage, i.e., the arc length approximately constant regardless of a variation in welding current. If now a welding wire feed speed is slowed down to extend the arc length from L1 to L2, then the arc voltage increases to shift the operating point from S1 to S2. However, since the welding current sharply decreases in accordance with this shift as shown in the figure, the quantity of fusion of the welding wire reduces to automatically compensate for the reduction in feed speed for the restoration of the arc length to the original length L1, so that satisfactory welding can be continued. If a power source of the constant current characteristic shown in FIG. 3A is used, then the welding current scarcely varies even when the operating point shifts to S2. Therefore, the quantity of fusion of the welding wire does not reduce, as a consequence of which the reduction in feed speed cannot be compensated and arc disappears, resulting in disabled welding.
Generally, in CO.sub.2 arc welding, importance is attached to the stability of arc in a short-circuiting transfer region, and therefore, the quantity of inductance of a coil appropriate for CO.sub.2 arc welding must be made smaller than the quantity of inductance of manual welding. Furthermore, the quantity of inductance of the coil must be made as large as possible in manual welding in order to secure the stability of arc.
Therefore, in the prior art, the cross-sectional area of an iron core 13a of the constant current characteristic coil 13 is made greater than the cross-sectional area of an iron core 14a of the constant voltage characteristic coil 14, and the number of turns of the coil 13 of the constant current characteristic coil 13 is also made greater than the number of turns of the coil 14 of the constant voltage characteristic coil 14. As described above, in the prior art, two types of coils having different inductance values have been provided for different welding methods and they are connected across the output terminal 8 and the terminal 10 and across the terminal 9 and terminal 10, respectively.
However, since the magnitude of the inductance of the constant current characteristic coil 13 appropriate for manual welding is different from the magnitude of the inductance of the constant voltage characteristic coil appropriate for CO.sub.2 arc welding in the prior art arc welding apparatus, the constant current characteristic coil 13 for manual welding and the constant voltage characteristic coil 14 for CO.sub.2 arc welding have been wound around the separate iron cores 13a and 14a, respectively. Furthermore, the constant current characteristic coil 13 has been connected to the constant current characteristic output terminal 8, while the constant voltage characteristic coil 14 has been connected to the constant voltage characteristic output terminal 9. The above arrangement has led to the problem that the amount of use of iron core as well as the amount of use of coil are increased to cause an increase in manufacturing cost, an increase in weight of the arc welding apparatus and so on.